The wavelength plates (or wave plates) have been widely used for attaining high contrast ratios and improving color shift phenomena at wide view angles in color TFT liquid crystal displays of various kinds of display modes, and the like. For prevention of change in color or discoloration, it is needed to make the wavelength plate have a controlled wavelength dispersion of retardation (phase difference), i.e. that the properties of the wavelength plate be constant regardless of the wavelength. Thus, with respect to the wavelength dispersion of retardation, it is necessary that the retardation in the longer wavelength region is larger than the retardation in the shorter wavelength region, namely, the retardation is reverse dispersion of wavelength dispersion.
For example, JP-A-2004-70344 (“JP-A” means unexamined published Japanese patent application) discloses that a retardation sheet with a retardation in the film in-plane direction of reverse wavelength dispersion is extremely low in fluctuation of chromaticity and brightness, and thus it can be used as an excellent polarizing plate with a brightness-improving film. Further, for example, JP-A-2005-289980 discloses that it is possible to improve brightness characteristics and view angle characteristics drastically, by using a retardation sheet of reverse wavelength dispersion having a retardation in the film thickness direction.
However, generally, the wavelength dispersion of films is normal wavelength dispersion, in which the phase dispersion in longer wavelength region is normally smaller than the phase dispersion in shorter wavelength region, and thus, it is difficult to solve the problems above.
To solve the problems above, a method was developed: by bonding a quarter-wavelength plate having a retardation of in-plane birefringent light of ¼ wavelength and a half-wavelength plate having a retardation of birefringent light of ½ wavelength to each other, with their optical axes in the crossed state (see, e.g., JP-A-10-68816). However, for producing the above retardation sheets, a complicated process is required for controlling the optical directions (optical axes and slow phase axes) of the two polymer films.
Also, a method was proposed, which is to apply a liquid crystal composition that shows reverse wavelength dispersion by mixing a rod-shaped liquid crystal with a molecule orienting itself in the direction perpendicular to the major axis of the rod-shaped liquid crystal (see, e.g., JP-A-2002-267838). However, it is very difficult to control the wavelength dispersion arbitrarily by the method, because the mixed molecule is not liquid crystalline and thus, increase of the blending ratio leads to disappearance of liquid crystallinity.
Also, for solving the problems above, a method was disclosed, which utilizes a film that is prepared by applying a polymerizable liquid crystal compound with reverse wavelength dispersion on an oriented film, orienting and immobilizing (see, e.g., JP-A-2005-289980). The method is a favorable method, because it demands no film lamination, is simple to practice, and allows reduction of film thickness. However, the synthetic route for the compounds disclosed in JP-A-2005-289980 is relatively long, and thus, the method is not favorable, considering its production cost.